In batteries such as lithium primary batteries and lithium secondary batteries, lithium metal or a graphite intercalation compound in which lithium is intercalated, is used as a negative electrode active material, due to its low potential and high capacity. Lithium metal has an electric capacity of 2060 mAh/cm3; and the above graphite intercalation compound, with a composition of C6Li, has an electric capacity of 850 mAh/cm3. However, in recent years, devices with built-in batteries (e.g., cellular phones, personal digital assistants, laptop computers, video cameras, portable game consoles) are required to give higher performance and have longer battery runtime. Demands are also increasing for higher energy density in batteries.
In view of the foregoing, being considered are non-aqueous electrolyte batteries, etc. which use a negative electrode active material higher in capacity than lithium metal and the above graphite intercalation compound, examples of such an active material including magnesium metal (electric capacity: 3830 mAh/cm3) and calcium metal (electric capacity: 2070 mAh/cm3). If such batteries are realized, one can expect a higher energy density compared to batteries which use lithium metal as a negative electrode active material. To use magnesium metal as a negative electrode active material, it would be necessary to use a non-aqueous electrolyte including magnesium ions at high concentrations and thus having high ion conductivity.
On the other hand, electrochemical devices such as batteries and capacitors use, as an electrolyte, a non-aqueous electrolyte comprising a non-aqueous solvent and a supporting salt dissolved in the non-aqueous solvent. In the non-aqueous electrolyte, the supporting salt dissociates into cations and anions, and these ions (cations, in particular) become charge carriers in a battery reaction. Exemplary electrochemical devices include lithium primary batteries (e.g., lithium manganese dioxide batteries, lithium carbon fluoride batteries), lithium secondary batteries, and lithium ion capacitors. In these electrochemical devices, ions which act as charge carriers (hereafter, also simply referred to as carrier ions) are mainly lithium ions.
In electrochemical devices which use lithium ions as carrier ions, a lithium salt is used as the supporting salt. Widely used in particular, are lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium trifluoromethylsulfonate (LiCF3SO3), and lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2).
Although lithium ions are monovalent cations, if polyvalent cations such as magnesium ions are used as carrier ions, the amount of charge carried would be twice as much, or more, even when the concentration of the carrier ions remain the same in the non-aqueous electrolyte. Alkaline earth metal ions such as magnesium ions are divalent cations, and their use as carrier ions are expected due to their comparatively small formula weight. If the supporting salt has a small formula weight, the amount of the non-aqueous electrolyte used could be made smaller, and thus, the volume occupied by the electrodes could be made larger, thereby facilitating increase in energy density.
Anions of the supporting salt comprise non-metal elements, examples of such anions including hexafluorophosphate ion (PF6−), tetrafluoroborate ion (BF4−), perchlorate ion (ClO4), and trifluoromethanesulfonylimide ion (N(SO2CF3)2−); and such anions facilitate making the formula weight comparatively smaller and serve to increase energy density in electrochemical devices. For example, Mg(ClO4)2, Mg(N(SO2CF3)2)2, and the like dissolve comparatively easily in a non-aqueous solvent. However, combining these with polyvalent cations would cause the supporting salt to include anions in an amount twice as much, or more, compared to when a lithium salt is used; and as a result, the formula weight of the supporting salt would increase. As such, for reducing the formula weight of the supporting salt, selecting what kind of anion to use is also important.
If anions can comprise one non-metal element, and not two or more non-metal elements as with hexafluorophosphate ions, perchlorate ions, and the like, the formula weight could be made smaller. Examples of such non-metal anions include halogen ions such as chloride ion and bromide ion.
In Patent Literature 1, it is reported that when a liquid electrolyte comprising 2-methyltetrahydrofuran and magnesium bromide (MgBr2) dissolved in the 2-methyltetrahydrofuran is used, magnesium metal in the negative electrode is dissolvable and depositable.
Chloride ions have a smaller formula weight compared to bromide ions, and are therefore presumed to easily reduce the amount of the non-aqueous electrolyte. Magnesium chloride (MgCl2) has a formula weight of 95.2, being about the same as the LiBF4 formula weight of 93.7 and smaller than the LiPF6 formula weight of 151.9. However, an alkaline earth metal chloride such as magnesium chloride has low solubility in an organic solvent, and almost does not dissolve at all in a carbonate or the like typically used as a non-aqueous solvent.
Patent Literature 1 proposes a polymer electrolyte comprising polyethylene glycol and δ-type magnesium chloride uniformly dissolved in the polyethylene glycol.
Patent Literature 2 proposes a magnesium-ion-containing liquid electrolyte comprising magnesium chloride and an aluminum complex; and suggests that the aluminum complex acts to turn the magnesium chloride into binuclear complex ions for dissolution.